Recently, power supply devices which switch DC power using switching elements are popularly used for driving semiconductor light-emitting elements such as light-emitting diodes. These power supply devices include a dimming function for controlling the amount of light emitted by a light-emitting diode in accordance with an externally input dimming signal.
Conventionally, as disclosed in, e.g., JP-A 2003-157986 (KOKAI), a power supply circuit includes both a voltage dimming circuit that controls an applied voltage to a light-emitting diode, and a duty dimming circuit that switches on and off the applied voltage to the light-emitting diode. A dimming control signal controls switching between the voltage dimming circuit and duty dimming circuit.
In the power supply circuit disclosed in JP-A 2003-157986 (KOKAI), dimming control is carried out based on a pulse width. Therefore, the light output from the light-emitting diode may flicker. Also, a switch element connected in series or parallel to the light-emitting diode is required in addition to a current-limiting element for output current control based on the pulse width control. Therefore, the number of components of the power supply circuit is increased, and the efficiency of the power supply circuit drops.
However, since a light-emitting diode exhibits nearly constant voltage characteristics, a component or circuit having a current-limiting element is required to stably light on the light-emitting diode. In general, when the light-emitting diode is controlled by a power supply device using a switching element, current control is adopted. This current control is an important control element in design of a lighting device since the temperature of the light-emitting diode is decided by a current value to be supplied to the light-emitting diode, and it influences a service life of the element.
Dimming the light-emitting diode is relatively easily attained compared to a conventional electric discharge lamp lighting device. This is based on the fact that the light-emitting diode as a load has electrically stable characteristics, and suffers less variations of its characteristics against an external factor such as a temperature. However, when constant current control is adopted in an application that requires deep dimming, the light-emitting diode can be stably lighted in a region with a large full lighting current, but a current detection signal or a current reference value required to control this current detection signal becomes a very low signal in a deep dimming region. Therefore, a detection circuit or comparator, which detects a current, is required to have high precision, and it becomes difficult to attain stable operation due to high susceptibility to noise. Hence, a signal voltage required for control may be increased. However, in general, a current detection signal is detected by a resistor inserted in series to the light-emitting diode, and consumption power and heat generation amounts by this resistor increase in a region where a current flowing through the light-emitting diode is large, thus disturbing development of the power supply device.
As a proposal which solves these problems, a method of constant-controlling an output voltage has been proposed. For example, an ON voltage of a light-emitting diode is higher than that of a general silicon diode. For example, in a GaN diode represented by a blue light-emitting diode, current begins to flow from about 2.5 V, and a full lighting state requires a voltage as low as about 3.5 to 4.5 V. Even for deep dimming, relatively stable dimming can be attained without being influenced by the performance of the light-emitting diode or noise. However, since a forward voltage of the light-emitting diode has negative temperature characteristics, the light-emitting diode is self-heated by a flowing current. As a result, since the forward voltage lowers and a current further increases, a heat generation amount may become larger, thus causing thermal runaway. Also, light-emitting diodes have large variations, and output currents vary due to individual differences of light-emitting diodes even when the output of a lighting device is adjusted. This problem is posed similarly in a dimmed state in addition to a full lighting state. As a result, the currents flowing through light-emitting diodes vary, resulting in variations of light outputs.